This invention relates to weighing scales and, more specifically relates to high speed scales which are both highly accurate and resistant to external vibrations.
A common industrial use of weighing scales is the measurement of predetermined portions or amounts of products such as food items to be placed in bags or similar closed packages. Multiple scales are often included in a single combination weighing machine. In such a combination weighing machine, multiple portions of product are weighed by multiple scales and various ones of the portions are selected and combined for placement into a single package. A single combination weighing machine typically includes eight to sixteen separate weighing scales. Examples of such combination weighing machines are disclosed in U.S. Pat. No. 4,538,693. The overall accuracy of a combination weighing machine can be no greater than the combined accuracy of the individual weighing scales. Moreover, the speed of operation of a combination weighing machine can be limited by the speed with which the individual weighing scales produce a measurement.
In some uses weighing scales can be subject to significant amounts of vibration. For example, a combination weighing machine such as that described includes multiple hoppers with pneumatically operated doors and other mechanically driven elements. The rapid closure of such doors or the operation of other mechanical elements commonly causes vibrations which are transmitted to the scales. Further, combination weighing machines are commonly located above a bagging machine which forms a bag from raw stock, places the output of the weighing machine into the bag, and seals the bag. Bagging machines may create vibrations which are transmitted to the weighing machine and its scales even if the weighing machine is anchored to the plant floor separately from the bagging machine. The vibration problem is aggravated when the weighing machine is mounted on top of the bagging machine rather than being separately anchored, but such mounting can significantly reduce the amount of plant floor space required to perform the weighing and bagging operations. A further vibration problem arises from the rapid filling of the scale bucket with the product to be weighed.
Weighing scales have previously been known which overcome the vibration problem to a large degree. They have been referred to as "balanced mass scales." Such scales are disclosed in U.S. Pat. Re. Nos. 27,056 and 3,347,328. In such scales, the center of the mass moment of the weighing elements in the horizontal plane is located in the vertical plane passing through the counterbalance pivot and the center of the mass moment of the weighing elements in the vertical plane is located in the horizontal plane which passes through the counterbalance pivot. The scales described in the two referenced patents included as the principal spring elements flexures which were used in the counterbalance pivot, the coupling between the weigh bucket load and counterbalance elements, and the structure used to restrain the load element from horizontal motion while permitting it to move vertically.
While such scales have proven satisfactory in the past, it has been found desirable to develop scales having increased accuracy and speed. Most notably, the accuracy of prior scales was limited because of the creep and hysteresis effects in the springs or flexures used in the scales.